1. Field of the Invention
This invention relates to liquid crystal compositions comprising compounds having a silacyclohexane ring, and liquid crystal display devices containing the same.
2. Description of the Related Art
Liquid crystal display devices are devices utilizing the optical anisotropy and dielectric anisotropy possessed by liquid crystal materials. According to the display mode, there are various types including the TN (twisted nematic), STN (super-twisted nematic), SBE (super-twisted birefringent effect), DS (dynamic scattering), guest-host, DAP (distortion of aligned phase) and OMI (optical mode interference) types. The most common display devices are ones having a twisted nematic structure on the basis of the Schutt-Hellfritt effect.
Although the properties required of the liquid crystal materials used in these liquid crystal display elements more or less vary according to the display mode, a wide liquid crystal temperature range and stability to moisture, air, light, heat and electric fields are commonly required for all display modes. Moreover, it is desired that the liquid crystal materials have low viscosity and provide a short addressing time, a low threshold voltage and high contrast in the cells. At present, there is no single compound that meets all of these requirements, and it is a matter of fact that liquid crystal mixtures obtained by mixing several or several tens of liquid crystal compounds and/or latent liquid crystal compounds are being used. Accordingly, it is also important that these components are easily miscible with each other.
Among the various display modes, the active matrix (AM)-driven twisted nematic (TN) mode using a thin film transistor (TFT) element array or thin film diode (TFD) element array is being extensively employed owing to its high image display quality (i.e., high fineness, high contrast and high response speed). On the other hand, the simple matrix-driven STN mode has achieved an improvement in display quality owing to the development of new driving methods such as the multi-line selection method, and is finding its own applications.
One of the cell design factors directly affecting the quality of display is retardation (R=d.times..DELTA.n in which d is the cell gap size and .DELTA.n is the refractive index anisotropy of the liquid crystal material). In the TN mode, the first minimum of transmittance at which R=0.4-0.5 is chosen where visual angle characteristics are preferentially considered, and the second minimum of transmittance at which R=0.8-1.0 is chosen where contrast is preferentially considered. On the other hand, a value of R in the range of 0.8-0.9 is chosen in the STN mode.
Consequently, it is required that the .DELTA.n of the liquid crystal material be in the range of 0.08-0.09 for the first minimum of transmittance and in the range of 0.11-0.20 for the second minimum of transmittance and the STN mode.
In recent years, there is a growing demand for AM-driven TN mode liquid crystal panels predominantly using the first minimum of transmittance, and it is strongly desired to lower the driving voltages of liquid crystal materials, as well as the driving voltages of driver ICs. Conventional liquid crystal materials for use by low-voltage driving have the disadvantage that they show a rise in viscosity and hence a decrease in response time.
In the case of AM driving, it is required from reasons characteristic of this driving method that liquid crystal materials have what is called a signal voltage holding property. This signal voltage holding property designates the degree of drop of the signal voltage applied to picture elements containing a liquid crystal material, within a given frame period. Accordingly, when the signal voltage does not drop (i.e., when the voltage holding ratio is 100%), the alignment of the liquid crystal molecules is not broken and, therefore, no reduction in contrast is caused. Moreover, this voltage holding property is affected by the environment in which the liquid crystal panel is used, so that the lifetime of the property tends to be shortened in an environment exposed to high-intensity light as in liquid crystal panels for projection use, and in an environment exposed to high temperatures as in liquid crystal panels for use on automobiles. The threshold voltage of a liquid crystal material can be lowered by increasing its dielectric constant anisotropy (.DELTA..epsilon.). However, liquid crystal materials having high .DELTA..epsilon. have the disadvantage that they fail to achieve a high voltage holding ratio.